The present invention relates to a medium processing apparatus which reads information such as magnetic characters or image information carried on sheet-shaped mediums such as checks, while conveying the sheet-shaped mediums along a conveying path sheet by sheet, and more specifically, to a medium processing apparatus which can prevent a variation in conveying speed of sheet-shaped mediums such that the carried information of the sheet-shaped mediums can be accurately read.
In banking facilities such as banks, closed checks and bills (such as stocks and bonds) are put into a check reading apparatus so as to read surface images and magnetic ink characters on the checks and bills. Then, the checks and bills are sorted depending on the read results. Recently, as electric payments are spread, the read image data and magnetic ink characters are processed by computers so as to manage the checks and bills. Patent Documents 1 and 2 disclose such a check reading apparatus.
As disclosed in Patent Documents 1 and 2, a check conveying mechanism of the check reading apparatus transmits the torque of a conveying motor to a plurality of conveying rollers, disposed along a check charring path, through an endless belt and carries checks along the conveying path while sequentially transferring the checks from an upstream conveying roller to a downstream conveying roller.
The checks are conveyed while the magnetic ink characters and image data are read by a magnetic head and an image sensor disposed in the conveying path. When the conveying speed of the checks is varied during the reading operation, a detection signal pattern obtained from the magnetic head is also varied. As a result, the reading precision of magnetic ink character by the magnetic head is degraded. Therefore, the conveying speed of the checks needs to be constantly maintained during the reading operation.    Patent Document 1: Japanese Patent Publication No. 2004-206362A    Patent Document 2: US Patent Publication No. 2004/0257626 A1
In the conveying mechanism in which the endless belt is wound around the conveying rollers disposed along the conveying path in a tensile state so as to be driven by the conveying motor, when a tensile force acting on the endless belt is varied, a load acting on spindles of the conveying rollers around which the endless belt is wound is varied by the variation in tensile force such that a deflection amount of the spindles is varied. The tensile force of the endless belt is varied by a load generated when a check enters nip portions of the conveying rollers and pressure rollers. Further, as the endless belt is separated from the driven position by the conveying motor, variations in the load applied from the respective conveying rollers around which the endless belt is wound and loads acting on the spindles due the variations in load are sequentially added and amplified. Therefore, the tensile force is significantly varied in a portion of the endless belt separated from the driven position.
In the spindles of the conveying rollers of which portions where the variation in tension is large are wound by the endless belt, the deflection amount is significantly varied depending on the variation in tension. When the variation in deflection amount of the spindles of the conveying rollers is large, the distance between shafts of the conveying rollers and the pressure rollers is varied, and a deflection amount of the belt and a driven amount of the conveying motor are out of proportion such that the check conveying speed (conveying pitch) is disturbed.
Meanwhile, when a check is fed to the nip portions of the conveying rollers and the pressure rollers, a reaction force is caused by a nip force so as to act in the reverse direction to the conveying direction with respect to the check. When the reaction force is large, the disturbance occurs in the check conveying speed.